Hi OP. I can only tell you what the head at our prep told me last year. Firstly, there is a 5 mark “margin of error” with CAT tests, so you have to factor this in. However, they are in the business of guiding children into the “right” schools year after year. They track CAT scores from reception and what they generally find is this -
SPGS, SPS, G&L, LU & Westminster - you need to be 132 plus on average. SPGS particularly, more like 135.
Hampton, LEH and WHS - about 128-130. Not because they are less academic once you get in, but just because the Central London schools are more competitive entry due to numbers applying.
PHS and KGS - about 125.
Ibstock etc I can’t remember, but slightly lower.
So yes, It’s crazy and it’s a bubble. Averages in SW London are totally skewed and if you’re not in the top 2% nationally, you will not get into the SPS, KCS, G&L,or LUs etc. Hundreds of children who are at this level, CAT score-wise, will not get in anyway, even though they do more than adequately in the exams / interviews because there simply aren’t enough places to take all the DC who would actually cope well in these schools. This is why you have to sit 5 schools to raise the odds.